1. Technical Field
The present invention generally relates to the linearization of nonlinear optical transmission systems by the generation of a compensation signal and, more particularly, to a method and apparatus for compensating the nonlinearities of an optical transmission system including at least one of a semiconductor laser, an optical amplifier, and an optical fiber communications link by the generation of a postdistortion signal.
2. Description of the Prior Art
Optical communications system which include a semiconductor laser, an optical fiber communication link, and an optical receiver are well known in the art. These communications systems are adapted to carry a wide range of information including voice, video and data.
The typical optical communications system includes a laser transmitter which transduces an electrical information signal into an optical signal. The optical signal is then carried over an optical fiber communications link where it is converted back to an electrical signal by a photodetector of an optical receiver. The transmission scheme may be analog or digital and the modulation scheme amplitude, phase, or frequency, or any combination of the above.
One of the most advantageous optical communication systems from the viewpoint of simplicity and bandwidth considerations is an analog scheme where the optical intensity of the semiconductor laser is amplitude modulated. The optical transmission system, including the semiconductor laser, optionally an optical amplifier, and optical fiber communications link is required to convert the electrical information signal linearly into an optical signal and to transmit the optical signal linearly over the communications link. In general, distortions caused by the semiconductor laser, the optical amplifier, and the fiber optic communications link cause the system to operate in less than an optimum manner. Increasingly, this type of optical communication system is playing an important role in the delivery of high quality signals in all types of CATV architectures.
Distortion in optical transmission systems can originate from several different sources. One of the primary sources is the electrical to optical transducer, a laser diode in most systems. Another contributor is the optical communications link and, more recently, any optical amplifier in the optical link. Some of these sources produce similar distortion signals which may even cancel others, but usually each distortion has its own unique characteristics and should be compensated for independently.
A laser diode generally exhibits distortion from several identifiable causes. The first is generally that caused by the non-linearities of its LI (light intensity as a function of electrical current) transfer function. The nonlinearity may be superlinear (L increases at increasing rates for increases in I), or it may be sublinear (L increases at decreasing rates for increases in i). This type of laser diode distortion causes mainly second order and higher intermodulation products that are independent of frequency. This type of distortion can be corrected by generating a distortion signal which is generally similar (sum and difference beats of the carrier frequencies) with a similar amplitude but opposite phase. Another characteristic distortion of the laser diode is where the amplitude and phase of the modulating signal is distorted as a function of frequency changes. To correct for such a distortion, a compensation signal which varies in amplitude and phase as a function of frequency is advantageous.
The distortion generated by an optical link is generally caused by phase and amplitude dispersion. Generally, phase dispersion causes the different modulating frequencies of an optical signal to be phase shifted different amounts. Phase dispersion is proportional to the length of the optical link and causes second order distortion with amplitude proportional to distortion frequency. Present optical communications systems use a 1330 nm. optical wavelength signal to minimize phase dispersion. However, newer systems that operate at a 1550 nm. optical wavelength are capable of minimizing amplitude signal losses but with effect of increasing phase dispersion. It would be advantageous to compensate for the increased phase dispersion of the 1550 nm. system while maintaining its lower optical loss. Optical amplifiers, particularly Erbium doped fiber amplifiers (EDFA), allow greater lengths of the optical fibers to be used but produce their own distortion in the form of an amplitude versus frequency characteristic.
One technique of compensating for distortion from the above-described sources utilizes a predistortion signal to compensate the RF signal modulating the laser transmitter for the distortion in the optical communications system. Such predistortion is discussed in commonly assigned, copending application Ser. No. 805,251 entitled "Method and Apparatus for Predistortion" and application Ser. No. 805,259 entitled "Method and Apparatus for Predistortion", which are incorporated herein by reference. However, predistortion networks cannot generally compensate for distortion which may be unique to a particular receiver or receiver location. For example, distortion caused by fiber dispersion is dependent upon fiber length and thus may be different at different receiver locations serviced by the same transmitter. Additionally, the receiver itself may introduce distortion which may vary from receiver to receiver.